1. Field of the Invention
The present invention relates to camera film drive motor control devices, and more particularly to such control devices which electrically detect variations in current flow through the motor for purposes of inversion or stop control.
2. Prior Art
Camera film wind-up, when performed electrically with a motor, requires current as large as approximately 1A to be normally supplied to the motor. This current level makes the load too large for batteries used in an ordinary compact camera as a film drive motor power source. Particularly when the motor rotation is forced to stop due to film over-tension of the film with film wind-up after completion of exposure on the last frame, the motor current becomes more than double the motor current during the normal motor rotation. This state, not corrected or removed, unnecessarily reduces the life of the camera batteries. Therefore, to minimize battery power consumption, it is necessary to immediately cut off the power supply to the motor when the motor rotation is forced to stop. Such forced stopping of motor rotation occurs also in the case of failure of a normal film wind-up operation due to an accident.
U.S. Pat. No. 3,946,409 discloses a film drive motor control device which, based on the fact that motor current varies with the variation in load to the motor, detects the load increase as voltage variations across a series connected resistor to cut off the power supply to the motor. As shown in FIG. 1, resistor 2 is connected in series to film wind-up motor 1 to apply the voltage developed at a node between resistor 2 and motor 1 to the emitter of transistor 3. Moreover, a predetermined level of voltage is applied through the intermediate terminal of variable resistor 4 to the base of transistor 3. The predetermined voltage level allows transistor 3 to turn on with the emitter potential kept sufficiently lower than the base potential when a normal film wind-up is effected, while turning off transistor 3 with a rise in the emitter potential due to the motor current increase caused by the forced stop rotation of motor 1. When transistor 3 turns off, relay 6 is excited to open switch 5 connected in series to motor 1, whereby the current flow to motor 1 is cut off.
In this prior art circuit arrangement, the electric power which is consumed by resistor 2 is unnecessary for the purpose of the motor drive. Especially when the circuitry is driven with such a low-voltage power source as 3V, a voltage drop caused by resistor 2 is to be minimized, e.g., by reducing the resistance value of resistor 2 in order to make the most of the power source. When, however, a battery whose internal resistance increases as its capacity drops, such as when an AA size manganese battery is used, the available motor current range is limited if the circuit arrangement is required to ensure the use of both a brand new battery and an old one consumed to its operative limit. For example, the motor current during a normal film wind-up is approximately 0.8A with a new battery and approximately 0.5A with an old one consumed to its operative limit. In contrast thereto, when the motor rotation is forced to stop, the motor current becomes 3.0A with the new battery and 1.0A with the old one. In this case, the respective circuit constants of resistors 2 and 4 and transistor 3 should be selected so that they may discriminate 0.8A from 1.0A motor current for the detection of stopping the motor rotation.
Since, however, the range of emitter potential variations becomes narrower as the voltage drop caused by resistor 2 is reduced, as described earlier, transistor 3 is required to have a sharp switching characteristic so that it can respond to a slight emitter potential variation. As is well known, however, the switching characteristic of a single transistor is not sharp enough. Additionally, the threshold level for the switching operation of a transistor varies with the temperature. Accordingly, these switching and temperature characteristics of the transistor necessarily determine the acceptable lower limit of the voltage drop caused by resistor 2, whereby developing any lower voltage drop by resistor 2 would make the prior art circuit arrangement unreliable and improper for practical use.